1. Field of the Invention
The present invention relates generally to a piezoelectric sensor for monitoring kinetic momentum of movable mechanical constructions. More specifically, the invention relates to a piezoelectrio-type momentum sensor suitable for use as an accelerometer, such as employed for an automotive control system. Particularly, the invention relates to a piezoelectric sensor specifically adapted for monitoring relative displacement between a vehicle body and a wheel axle in an automotive suspension system.
2. Description of the Background Art
U.S. Pat. No. 4,696,489, issued on Sep. 27, 1987, to Takeshi FUJISHIRO et al, and assigned to the common assignee of the present invention discloses an automotive suspension control system in which an accelerometer is incorporated for monitoring vertical acceleration of the vehicle body for utilizing the monitored vertical acceleration of the vehicle body as one of control parameter in the damping characteristics of the vehicular suspension system. In the construction shown in the aforementioned U.S. Patent, the accelerometer employs an inertia member causing deformation of a resiliently deformable member and a strain gauge for detecting deformation magnitude and speed and thereby detecting the vertical acceleration of the vibrating vehicle body. Such a type of accelerometer may be replaced with a piezoelectric-type accelerometer for performing the same or a similar acceleration monitoring operation.
One typical piezoelectric-type accelerometer which employ piezoelectric element as a sensor element, has been disclosed in the Japanese Patent First (unexamined) Publication (Tokkai) Showa 59-23223, for example. Such a conventional piezoelectric accelerometer employs a piezoelectric sensor element supported in a housing in cantilever fashion. In this construction, the piezoelectric sensor element may vibrate about the supported end when vibration energy is exerted. This concentration of distortion stress around the supported end causes uneven exhaustion. Therefore, such concentrated stress may make it difficult to stably maintain reasonable performance and will shorten the life of the accelerometer.
In addition, when the supporting structure, supporting the piezoelectric sensor element is of asymmetric construction, the sensitivity of the sensor element tends to fluctuate, depending upon the exerting direction of the vibration energy. In order to avoid this, the conventional cantilever type piezoelectric sensor element supporting structure is precisely symmetric in construction. This requires high accuracy machining, such as by means of laser machining apparatus. As will be clear, such high accuracy machining requires a substantial cost.
Therefore, an improvement has been proposed in co-pending U.S. patent application Ser. No. 120,964, filed on Nov. 16, 1987, by Hiroshi KOBAYASHI et al. In the proposed improvement, the piezoelectric kinetic momentum sensor employs a thin oscillable diaphragm member on which a thin plate form piezoelectric sensor element is mounted. The overall circumferential edge of the oscillable diaphragm member is supported on a sensor casing. So as to avoid fluctuation of sensitivity, depending upon the exerting direction of a kinetic energy, the diaphragm member and the piezoelectric sensor element are formed in a coaxial thin disc shaped configuration, in the preferred construction. In the further preferred construction, the sensor casing is composed of separable two bottomed cylindrical components, each of which has a circumferential cylindrical wall section having free edge mating with the other. The circumferential edge of the diaphragm member is sandwiched between the mating free edges of the cylindrical components to define a sealingly enclosed internal space within the sensor casing.
Furthermore, the above-identified co-pending U.S. Patent Application proposes, in FIG. 5 of the application, use of mass weight member fitted to the piezoelectric element for adjustment of the resonance frequency and providing maximized sensitivity of kinetic energy is employed.
In the last mentioned piezoelectric kinetic momentum sensor, it is required to minimize the weight of the mass weight member with maximized sensitivity and accuracy in monitoring the kinetic energy.